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PostPosted: 06 Apr 2017, 21:44 
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Hi, I have spronged hidden wires in the past...usually with a drill though. It would be nice to think one of these residence related issues would be the solution, but more than two locations and builds are involved. A new set of amps is being done with other components to see if it will have the same problems. More to follow. I would really hate to have to stop doing class A KT120 amps for lack of good tubes. It would force me to do something radical like water cooled CX4000As in push pull. (only after a major power upgrade to the house though :D )

Good listening
Bruce

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PostPosted: 07 Apr 2017, 03:04 
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Pffffft! Why so small? :D

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PostPosted: 07 Apr 2017, 07:45 
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OK - let's cut directly to the chase, as this last exchange is intriguing!

Do either of the problem children have access to a Dummy Load? Has that been tried?

Guys and gals, for the purposes of this, and only this, effort - I would be willing to send the fixin's for one (1) dummy load (some assembly required) anywhere the North America or Europe, no cost, and including shipping (but not customs). It will consist of a handful of high-wattage resistors to be configured at 'that' end for the specific use.

But, a Dummy Load is stable and will allow testing the amp to its limits from various sources and with various input voltages. Could be revealing.


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PostPosted: 07 Apr 2017, 17:12 
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Hi, Access to or use of a dummy load to do testing will not stop the behavior when hooked up to speakers. Thus far everything points to sub par tubes.

Good listening
Bruce

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PostPosted: 07 Apr 2017, 19:02 
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gofar99 wrote:
Hi, Access to or use of a dummy load to do testing will not stop the behavior when hooked up to speakers. Thus far everything points to sub par tubes.

Good listening
Bruce

This sounds right. How often do a get a run of a faulty parts. Should be easy enough to check against tubes which work perfectly elsewhere.

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PostPosted: 10 Apr 2017, 03:17 
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We have tried with tubes that worked perfectly elsewhere. I tried some JJ KT88s. They worked perfectly at 38 watts in another amp. In these amps they arced on the standard KT88 setting and redplated even when dissipation was reduced below the 38 watts they had handled perfectly previously.

My money is on some oscillation possibly at a frequency beyond the limits of my scope, possibly the CCS. It clearly doesn't get through the output transformer as everything is beautifully clean-square and sine-beyond the design parameters of the amp. and I can't even excite the resonant frequency of the transformer when passing a signal of that frequency. But first things first. Let's see how the new build goes. I will test the transformer impedance before building. If the problem persists I may even try a MOSFET ccs solution as that has been shown to be much less oscillation prone than a 317 based one.


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PostPosted: 10 Apr 2017, 08:21 
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Juancho wrote:
I may even try a MOSFET ccs solution as that has been shown to be much less oscillation prone than a 317 based one.


I use MOSFET ccs in some of my preamps - RF design protocol is required or they will oscillate easily... at 400MHz!

Cheers!

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PostPosted: 10 Apr 2017, 08:32 
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Juancho wrote:
We have tried with tubes that worked perfectly elsewhere. I tried some JJ KT88s. They worked perfectly at 38 watts in another amp. In these amps they arced on the standard KT88 setting and redplated even when dissipation was reduced below the 38 watts they had handled perfectly previously.

My money is on some oscillation possibly at a frequency beyond the limits of my scope, possibly the CCS. It clearly doesn't get through the output transformer as everything is beautifully clean-square and sine-beyond the design parameters of the amp. and I can't even excite the resonant frequency of the transformer when passing a signal of that frequency. But first things first. Let's see how the new build goes. I will test the transformer impedance before building. If the problem persists I may even try a MOSFET ccs solution as that has been shown to be much less oscillation prone than a 317 based one.


This is why I am suggesting running these problem children into a dummy load. If it remains stable into a non-reactive load, that would be revealing.


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PostPosted: 10 Apr 2017, 14:14 
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The current pair have been tested with a single driver speaker, so pretty close to a dummy load. It made not difference to the redplating. If the second pair show the same symptoms I will test them with a dummy load.


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PostPosted: 10 Apr 2017, 15:29 
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Juancho wrote:
The current pair have been tested with a single driver speaker, so pretty close to a dummy load. It made not difference to the redplating. If the second pair show the same symptoms I will test them with a dummy load.


Single-Driver speakers may vary from 4 < 8 < 40 ohms depending on the type and frequency. I would hardly define that as "pretty close" to anything. Typically, about half-nominal to 5 x nominal. A sample curve is linked below.

You want a truly non-reactive load.

https://upload.wikimedia.org/wikipedia/ ... ce.svg.png


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